We want to wait until we have read the response body before we log the request
as complete, otherwise a confusing thing happens where the request appears to
have completed, but we later fail it.
To do this, we factor the salient details of a request out to a separate
object, which can then keep track of the txn_id, so that it can be logged.
on_notifier_poke no longer runs synchonously, so we have to do a different hack
to make sure that the replication data has been sent. Let's actually listen for
its arrival.
Some streams will occaisonally advance their positions without actually
having any new rows to send over federation. Currently this means that
the token will not advance on the workers, leading to them repeatedly
sending a slightly out of date token. This in turns requires the master
to hit the DB to check if there are any new rows, rather than hitting
the no op logic where we check if the given token matches the current
token.
This commit changes the API to always return an entry if the position
for a stream has changed, allowing workers to advance their tokens
correctly.
